Spiral heat exchanger with anti-fouling properties

ABSTRACT

The invention relates to a spiral heat exchanger comprising a spiral body formed by at least one spiral sheet wound to form the spiral body forming at least a first spiral-shaped flow channel for a first medium and a second spiral-shaped flow channel for a second medium, wherein the spiral body is enclosed by a substantially cylindrical shell being provided with connecting elements communicating with the first flow channel and the second flow channel. At least a part of the spiral heat exchanger is provided with a coating comprising silicon oxide, SiO x .

TECHNICAL FIELD

The present invention refers generally to spiral heat exchangersallowing a heat transfer between two fluids at different temperature forvarious purposes. Specifically, the invention relates to a spiral heatexchanger which has been coated for improving anti-fouling propertiesand has in some embodiments been given predetermined, structuralproperties for ensuring that the coating remains on the product when theproduct is used.

BACKGROUND ART

Spiral heat exchangers are generally formed by winding two metal sheetsaround one another so as to delimit two separate passages. The two flatsheets are welded together at a respective end, wherein the welded jointwill be comprised in a center portion of the sheets. The two sheets arewound around one another to form the spiral element of the sheets so asto delimit two separate passages or flow channels. Distance members,having a height corresponding to the width of the flow channels, may bearranged on one or both of the sheets.

Two inlet/outlet channels are formed in the center of the spiralelement. The two channels are separated from each other by the centerportion of the sheets. A shell is welded onto the outer periphery of thespiral element. The side ends of the spiral element are processed,wherein the spiral flow channels may be laterally closed at the two sideends in various ways. Typically, a cover is attached to each of theends. The covers may include connection pipes extending into the centerand communicating with a respective one of the two flow channels. At theradial outer ends of the spiral flow channels a respective header iswelded to the shell or the spiral element forming an outlet/inlet memberto the respective flow channel

Such a spiral heat exchanger is described for example in U.S. Pat. No.5,505,255. In this case, the spiral heat exchanger is formed by windingtwo metal sheets each having protrusions on one of their faces.

In many industrial processes fouling of heat transfer equipment is ofconcern. In order to keep a satisfying performance of the equipmentregular service and cleaning is necessary to remove build up of depositson the heat transfer surfaces. The deposits arise e.g. from the fluidsin the equipment, microbial growth and/or dirt.

Spiral heat exchangers may over time get fouled due to a slow depositionof material on the surfaces of the sheets. Such so called crystallizedfouling leads to a decreased heat transfer and increased pressure drop,and results in an overall reduced performance of the spiral heatexchanger. Depending e.g. on the fluids used the spiral heat exchangermay be seriously fouled and difficult to clean, thus requiring strongdetergents and/or powerful mechanical cleaning over a substantial timeperiod in order to restore the performance of the heat exchanger. Thecleaning of spiral heat exchangers may both be time consuming andcostly. Also, the process to which the spiral heat exchanger isconnected, may have to be shut down during said cleaning.

The sheets of spiral heat exchangers are made of metal. The basematerial, i.e. metals used, may have a high surface free energy thatresults in most liquids easily wetting the surface of the sheets.

Also, when spiral heat exchanger sheets are produced the formingoperation thereof may increase the surface roughness which often isassociated with faster build up of fouling deposits.

Other types of heat exchangers have been described wherein surfaces ofthe heat exchangers have been coated with a coating possessinganti-fouling properties

WO2009034359 discloses provision of a coating to reduce bio-fouling ofsurfaces in aquatic environments wherein the coating is applied by useof Plasma Assisted Chemical Vapour Deposition (PACVD).

US20090123730 discloses a surface of a heat exchanger which is to besoldered by means of a flux, and said surface is in addition to the fluxalso provided with at least one more layer containing an additive. Theadditive is reacted in order to modify the surface during soldering.

WO2008119751 discloses production of a hydrophobic coating forcondensers wherein the coating comprises sol-gel materials based on e.g.silicon oxide sol.

JP2000345355 relates to improving corrosion resistance and discloses afilm consisting of 55-99 wt % SiO₂ and 45-1wt % ZrO₂ which film isformed using sol-gel processing.

US2006/0196644 discloses a heat exchanger provided with a hydrophilicsurface coating comprising a gel produced by sol-gel processing.

It would be desirable to find new ways to ensure less fouling of heatexchangers, especially spiral heat exchangers and their sheets in orderto keep the spiral heat exchangers running for longer time periods.Also, a reduced shut down time for processes where spiral heat exchangerare involved would be desirable.

A problem encountered with presently known anti-fouling coatings is thepoor wear resistance of the coatings in applications with abrasive heatexchanging media, e g sand or other particulate material which entersthe spiral heat exchanger with the heat exchanging fluids. Furthermore,cracks in the coating may occur due to torque and tension forces actingon the spiral heat exchanger sheets in applications under highpressures.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved spiral heatexchanger, which show a reduced fouling of the sheets. Another object isto obtain embodiments of a spiral heat exchanger which are wearresistant in abrasive environments and have high resistance againstformation of cracks.

This object is achieved by a spiral heat exchanger comprising a spiralbody formed by at least one spiral sheet wound to form the spiral bodyforming at least a first spiral-shaped flow channel for a first mediumand a second spiral-shaped flow channel for a second medium. The spiralbody is enclosed by a substantially cylindrical shell being providedwith connecting elements communicating with the first flow channel andthe second flow channel. The spiral heat exchanger is provided with acoating comprising silicon oxide, SiO_(x), having an atomic ratio ofO/Si>1, a content of carbon 10 atomic % and a coating layer thickness ofabout 5-60 μm, which coating was prepared by sol-gel processing andapplied to at least a part of the sheets.

The spiral heat exchanger is advantageous in that fouling of the sheetsis reduced significantly. By applying a coating composition comprisingsol-gel material with organosilicon compounds to the spiral heatexchanger sheets both the surface free energy and roughness is lowered,leading to reduction of fouling, less and easy cleaning of spiral heatexchanger sheets. Moreover, the sol-gel coated spiral heat exchangersheets of the invention exhibit an excellent wear resistance and have aflexibility that reduces the risk of cracks appearing in the coating.

Generally, not only a part of but one side or both sides of therespective sheets may comprise the coating.

According to a further aspect of the invention the sheets of the spiralheat exchanger have a thickness of 2-6 mm.

According to another aspect of the invention the layer thickness of saidcoating on the piral heat exchanger sheets is 5-30 μm, preferably 2-20μm,

According to yet another aspect of the invention the coating comprisingsilicon oxide, SiO_(x), has an atomic ratio of O/Si≧1.5-3, preferablyO/Si≧2-2.5.

According to still another aspect of the invention the composition has acontent of carbon ≧20-60 atomic %, preferably ≧30-40 atomic %.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of the invention will appearfrom the following detailed description of different embodiments of theinvention with reference to the accompanying schematic drawings, inwhich

FIG. 1 is a perspective view of an open spiral heat exchanger accordingto the present invention;

FIG. 2 is a schematic cross sectional view of a spiral heat exchangeraccording to the present invention, and

FIG. 3 is a schematic cross section of a sheet for a spiral heatexchanger comprising an anti-fouling coating according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

A commonly known spiral heat exchanger includes at least one spiralsheet extending along a respective spiral-shaped path around a commoncentre axis and forming at least two spiral-shaped flow channels whichflow channels are substantially parallel to each other. Each flowchannel includes a radially outer orifice, which enables communicationbetween the respective flow channel and a respective outlet/inletconduit and which is located at a radially outer part of the respectiveflow channel with respect to the centre axis, and a radially innerorifice, which enables communication between the respective flow channeland a respective inlet/outlet chamber, so that each flow channel permitsa heat exchange fluid to flow in a substantially tangential directionwith respect to the centre axis. The centre axis extends through theinlet/outlet chambers at the radially inner orifice. Distance members(not shown in FIG. 1), having a height corresponding to the width of theflow channels may be attached to the sheets or be formed on the surfaceof the sheets. The distance members or studs support the spiral bodyformed by the at least one spiral sheets and the inner surface of theshell to resist the pressure of the working fluids of the spiral heatexchanger 1.

In FIG. 1 is shown a perspective view of a spiral heat exchanger 1according to the present invention. The spiral heat exchanger 1 includesa spiral body 2, formed in a conventional way by winding two sheets 3 ofmetal around a retractable mandrel. The sheets 3 are provided withdistance members or supports 4 (not shown in FIG. 1) attached to thesheets 3. The distance members or supports 4 serve to form the flowchannels 5 a, 5 b between the sheets 3 and have a length correspondingto the width of the flow channels 5 a, 5 b. In FIG. 1 the spiral body 2only has been schematically shown with a number of wounds, but it isobvious that it may include further wounds and that the wounds areformed from the centre of the spiral body 2 all the way out to theperipheral of the spiral body 2. The spiral body 2 is enclosed by ashell 6.

The shell 6 is formed as a cylinder having open ends, the open endsbeing provided with a flange. Lids or covers 7 a, 7 b are provided toclose the shell 6 in each end. Connection elements 8 a, 8 b are attachedto the outer surface of the shell 6. The lids or covers 7 a, 7 b areprovided with connection elements 8 a, 8 b. The connection elements 8a-b and 9 a-9 b are typically welded to the shell 6 and the covers 7 a,7 b, and are all provided with a flange for connecting the spiral heatexchanger 1 to a piping arrangement of the system of which the spiralheat exchanger 1 is a part of. Other configurations of the connectionelements are also possible.

The spiral heat exchanger 1 is further provided with gaskets, eachgasket being arranged between the open ends of the shell, the spiralbody 2 and the lids or cover 7 a, 7 b. The gaskets serves to seal offthe different wounds of the flow channels 5 a or 5 b from each other toprevent that a medium in the flow channels to bypass wounds of flowchannels 5 a or 5 b and lowering the thermal exchange. The gaskets,which can be formed as a spiral similar to the spiral of the spiral body2, is then squeezed onto each wound of the spiral body 2. Alternativelythe gaskets are squeezed between the spiral body 2 and the lids orcovers 7 a, 7 b. The gaskets can also be configured in other ways aslong as the sealing effect is achieved.

FIG. 2 shows a schematic cross section of the spiral heat exchanger 1 ofFIG. 1 having a spiral body 2, connections 8 a, 8 b provided on thecovers 7 a, 7 b of the spiral heat exchanger 1 and connected to the flowchannels 5 a, 5 b, respectively, at the centre of the spiral body 2, andconnections 9 a, 9 b provided on the outer of the shell 6 of the spiralheat exchanger 1 and connected to the flow channels 5 a, 5 b,respectively. The coating used according to the present invention may bereferred to as a non-stick coating and makes it easy to clean the sheets1 a of a fouled spiral heat exchanger 2. The coated sheets 3 accordingto the present invention show a better heat transfer over time comparedto conventional spiral heat exchanger sheets since the latter ones getsfouled much quicker and thus decrease the heat transfer performance to alarger extent. The coating of the sheets also results in a much moreeven surface thus resulting in better flow characteristics. Also thepressure drop is reduced over time for a spiral heat exchanger accordingto the present invention in comparison with conventional spiral heatexchangers, since the buildup of impurities, microorganisms and othersubstances is not as pronounced.

The coated spiral heat exchanger 1 according to the present inventionmay easily be cleaned just using high pressure washing with water. Witha sheet 3 according to the present invention there is no need forextensive time consuming mechanical cleaning or cleaning using strongacids, bases or detergents.

According to the present invention the sheets 3 of a spiral heatexchanger 1 is coated with a composition comprising organosiliconcompounds using a sol-gel process. The organosilicon compounds arestarting materials used in the sol-gel process and are preferablysilicon alkoxy compounds. In the sol-gel process a sol is converted intoa gel to produce nano-materials. Through hydrolysis and condensationreactions a three-dimensional network of interlayered molecules isproduced in a liquid. Thermal processing stages serve to process the gelfurther into nano-materials or nanostructures resulting in a finalcoating. The coating comprising said nano-materials or nanostructuresmainly comprise silicon oxide, SiO_(x), having an atomic ratio ofO/Si>1, preferably an atomic ratio of O/Si≧1.5-3, and most preferablyO/Si≧2-2.5. A preferred silicon oxide is silica, SiO₂. The siliconoxideforms a three dimensional network having excellent adhesion to thesheets.

The coating of the present invention further has a content of carbonsuch as found in hydrocarbon chains. The hydrocarbons may or may nothave functional groups such as found in hydrocarbon chains or aromaticgroups, e g C═O, C—O, C—O—C, C—N, N—C—O, N—C═O, etc. Preferably thecarbon content is ≧10 atomic %, preferably ≧20-60 atomic %, and mostpreferably ≧30-40 atomic %. The hydrocarbons impart flexibility andresilience to the coating. The hydrocarbon chains are hydrophobic andoleophobic which results in the non-stick properties of the coating.

In FIG. 3 is shown a schematic drawing of a sheet 3 for a spiral heatexchanger provided with a siliconoxide sol gel coating 10. Between thesheet 3 itself and the siliconoxide coating 10 is an interface 11between the coating 10 and a metal oxide film of the sheet 3. Thecoating bulk that follows said interface is the siloxane network 12 withorganic linker chains and voids that impart flexibility to the coating10. The outermost layer of the coating 10 is a functional surface 13, ie a hydrophobic/oleophobic surface for fouling reduction.

By the combination of a durable and yet flexible coating, a sheet 3 fora spiral heat exchanger 1 is achieved which has excellent non-stickproperties and also is wear and crack resistant. The flexibility of thecoating is especially important in order to avoid cracking of thecoating when the sheets move in relation to each other.

In one embodiment of the present invention at least one sol comprisingorganosilicon compound is applied to the surface to be coated. Thesurface may be wetted/coated with the sol in any suitable way. It ispreferable for the surface coating to be applied by spraying, dipping orflooding. At least a part of one side of the spiral heat exchanger sheetis to be coated. Alternatively, all surfaces of at least one side of asheet which during use in a spiral heat exchanger would be in contactwith a fluid are coated. Also, at least one side of a spiral heatexchanger sheet may be entirely coated. Alternatively, both sides of thesheet may be coated. If both sides are coated, they may be partly orfully coated, in any combination. Naturally, more surfaces than thesurfaces intended to be in contact with fluid may be coated. Preferably,all surfaces in contact with a fluid giving rise to fouling are coated.

In another embodiment the method comprises a pretreatment of at leastthe surfaces on the heat exchanger sheets to be coated with at least onesol. This pretreatment is also preferably carried out by means ofdipping, flooding or spraying. The pretreatment is used to clean thesurfaces to be coated in order to obtain increased adhesion of thelatter coating to the heat exchanger sheet. Examples of suchpretreatments are treatment with acetone and/or alkaline solutions, e.g.caustic solution.

In another embodiment the method comprises thermal processing stages,e.g. a drying operation may be carried out after a pretreatment and adrying and/or curing operation is often necessary after the actualcoating of the sheet with said sol. The coating is preferably subjectedto heat using conventional heating apparatus, such as e.g. ovens.

The composition comprising SiOx is applied to a sheet 3 to be used in aspiral heat exchanger. The application of the composition is made bymeans of sol-gel processing. The resulting film of said composition onthe sheet is preferably between 1 and 30 μm thick. The thickness of thecoated film is important for the use in a spiral heat exchanger. A filmthickness below 1 μm is considered being not enough wear resistant sincethe sheets in a spiral heat exchanger in use are able to move slightlyin relation to each other. This slight movement causes wear on the filmand with time the coating will become worn down. Also the thickness ofthe film has an upper limit since the application of substances on theheat transfer sheets influences the heat transfer and thus theperformance of the spiral heat exchanger. The upper limit for theapplied film is preferably 30 μm. Thus, the film thickness of thesilicon oxide sol containing composition is 1-30 μm, preferably 1.5-25μm, preferably 2-20 μm, preferably 2-15 μm, preferably 2-10 μm andpreferably 3-10 μm.

The base material for the sheets may be chosen from several metals andmetal alloys. Preferably, the base material is chosen from titanium,nickel, copper, any alloys of the before mentioned, stainless steeland/or carbon steel. However, titanium, any alloys of the beforementioned or stainless steel is preferred.

EXAMPLES

In the search for prolonged operational time of off-shore equipment,tests were conducted on low surface energy glass ceramic coatings.

Two low surface energy glass ceramic coatings, Coat 1 and Coat 2, weretested and the results are presented below. Coat 1 is a silan terminatedpolymer in butyl acetate and Coat 2 is a polysiloxan-urethan resin insolvent naphtha/butylacetate.

Phase A

The analysis shows properties of the coatings concerning substratewetting and adhesion, contact angle, coating thickness and stabilityagainst 1.2% HNO₃ in H₂O, 1% NaOH in H₂O and crude oil. The results aresummarized below in Table 1.

TABLE 1 Coat 1 Coat 2 Substrate Excellent Excellent wetting SubstrateAl: 0/0 Al: 0/0 adhesion Stainless steel: 0/0 Stainless steel: 0/0 Ti:0/0 (see below) Ti: 0/0 (see below) Contact angle H₂O: 102-103° H₂O:102-103° measurements Coating 4-10 μm 2-4 μm thickness Stability 1.2%HNO₃ in H₂O: 1.2% HNO₃ in H₂O: 1½ h at 75° C. 1½ h at 75° C. 1% NaOH inH₂O: 1% NaOH in H₂O: 3 h at 85° C. 2 h at 85° C. Crude oil: 6 months atRT Crude oil: 6 months at RT

Both coatings showed excellent wetting when spray coated onto eitherstainless steel or titanium substrates.

Adhesion was determined by cross-cut/tape test according to DIN EN ISO2409. Rating is from 0 (excellent) to 5 (terrible). 0 or 1 is acceptablewhile 2 to 5 is not. First digit indicates rating after cross cut (1 mmgrid) and the second digit gives rating after tape has been applied andtaken off again.

To obtain the best adhesion for Coat 1 and Coat 2 the substratesrequired pre-treatment. The substrate is submerged in an alkalinecleaning detergent for 30 minutes. Afterwards the substrate is washedwith water and demineralized water and is dried before Coat 1 is appliedwithin half an hour to achieve the optimal adhesion. Tests have shownthat the adhesion is reduced if cleaning of the substrate is onlycarried out with acetone. Pre-treatment is also necessary for stainlesssteel substrates coated with Coat 2. This coating displayed unaffectedadhesion whether an alkaline detergent or acetone was used aspre-treatment. If the pre-treatment step is neglected or not madecorrectly it will affect coating adhesion.

Both coatings showed good stability under acidic condition. The coatingswere stable for 1 ½ hour at 75° C. and more than 24 hours at roomtemperature.

Under alkaline conditions Coat 1 showed a better result than Coat 2.Coat 1 could withstand the alkaline conditions for 3 hours at 85° C. andCoat 2 for 2 hours at 85° C. Both coatings showed no decomposition orreduction in oleophobic properties after being submerged in crude oil atroom temperature for 6 months.

Phase C

Coating of Spiral Heat Exchanger Sheets

Coat 1 and Coat 2 were applied to spiral heat exchanger sheets. Allsheets underwent pre-treatment which consisted of:

treatment with acidic and alkaline solutions to remove fouling,

high pressure washing of the sheets with water

The sheets were left to dry before application.

This pre-treatment was completed a day before Coat 1 and Coat 2 wereapplied to the sheets. Consequently, this procedure did not follow therecommended approach as outlined above. As the sheets had been left todry at ambient temperature, some sheets were still wet 15 sheet weretreated with Coat 1 and 15 sheets with Coat 2 by spray coating. Thecoating thickness was measured to be 2-4 μm and the coating was appliedon both sides of the sheets. Curing/drying was performed at elevatedtemperatures of 200° C. or 160° C. respectively for 1 ½ hour in anon-site oven. The spiral heat exchanger sheets were then assembled withthe remaining untreated 319 sheets. The coated sheets were placedrespectively in the front, middle and end of the assembled unit. Theevaluation of the coated sheets was performed after more than sevenmonths of operation.

Phase D

Determination of Content in Coating by XPS Analysis

Three different silicon oxide-coated Ti substrates were analyzed beforeand after use by means of XPS (X-ray Photoelectron Spectroscopy), alsoknown as ESCA (Electron Spectroscopy for Chemical Analysis). The XPSmethod provides quantitative chemical information—the chemicalcomposition expressed in atomic %—for the outermost 2-10 nm of surfaces.

The measuring principle is that a sample, placed in high vacuum, isirradiated with well defined x-ray energy resulting in the emission ofphotoelectrons. Only those from the outermost surface layers reach thedetector. By analyzing the kinetic energy of these photoelectrons, theirbinding energy can be calculated, thus giving their origin in relationto the element and the electron shell.

XPS provides quantitative data on both the elemental composition anddifferent chemical states of an element (different functional groups,chemical bonding, oxidation state, etc). All elements except hydrogenand helium are detected and the surface chemical composition obtained isexpressed in atomic %.

XPS spectra were recorded using a Kratos AXIS Ultra^(DLD) x-rayphotoelectron spectrometer. The samples were analyzed using amonochromatic Al x-ray source. The analysis area was below 1 mm².

In the analysis wide spectra were run to detect elements present in thesurface layer. The relative surface compositions were obtained fromquantification of detail spectra run for each element.

The following three samples were XPS analyzed:

1. Siliconoxide (new) on Ti-plate—coating on both sides.

2. Siliconoxide (used) on Ti-plate—coating on one side

3. Siliconoxide on DIN 1.4401 stainless steel plate, coating on bothsides.

The analysis was performed in one position per sample, except for sample1, where two positions were analyzed. The results are summarized inTable 2 showing the relative surface composition in atomic % and atomicratio O/Si.

TABLE 2 Sample O/Si C O Si N 1 new (pt 1) 2.25 61.1 23.5 10.5 4.2 2 new(pt 2) 2.30 61.0 23.9 10.4 4.1 2 used 2.29 68.0 19.5 8.6 3.1 3 1.46 41.934.3 23.4  (0.2)* *weak peak in detail spectra, signal close to noiselevel

As seen in Table 2 mainly C, O and Si were detected on the outermostsurfaces, i e 41.9-68.0 atomic % C, 19.5-34.3 atomic % 0 and 8.6-23.4atomic % Si.

Note that in the atomic ratios O/Si, the total amount of oxygen is used.This means that also oxygen in functional groups with carbon isincluded. Otherwise for silica, from theory is expected a ratio O/Si of2.0 for the bulk pure silica SiO₂.

Inspection During Operation

After four months of operation an off-shore pre-inspection bythermo-imaging was performed. Thermo-image of the mid region of heatexchanger in operation. The identity of the two coating systems waspresumed from the installation, but it was obvious that two sets ofspiral heat exchanger sheets show increased heat transfer compared tothe rest of the unit.

The inspection showed an elevation temperature at the coated sheets. Thenon-coated sheets showed a lower operating temperature. The differencein temperature is presumed due to reduced fouling, hence a higher crudeoil flow in the coated region which produces an elevated temperature.

Inspection of Sheets After Operation

The term fouling is used to describe the deposits formed on the sheetsduring operation. The fouling are residues and deposits formed by thecrude oil and consists of a waxy, organic part and a mineral/inorganicpart.

The visual inspection revealed that the sheets with the coatingdesignated Coat 1 was covered with the least amount of fouling on thecrude oil facing sheet side. Also, the other coating system designatedCoat 2 had a reduced amount of fouling on the crude oil facing sheetside compared to the bare titanium surface but to a lesser extent thenCoat 1

By subtracting the average weight of a clean sheet from the weightrecorded for the individual fouled sheets the average amount of foulingper surface type was calculated (table 3). Note, the weight of thecoating was not compensated for and so the real fouling reduction isslightly higher. If the coating is estimated to be pure SiO₂ (density2.6 g/cm³) then the amount of coating per sheet is about 20 g.

TABLE 3 Average Fouling Surface fouling* (g) STDEV reduction (%)Titanium 585 125 — Coat 1 203 48 65 Coat 2 427 144 27

For both coating systems the fouling of the sheets were more easilyremoved compared to the fouling adhering to the bare titanium surface,see Table 4. The difference in cleaning requirements was tested bymanually wiping of the sheets with a tissue and by high pressure watercleaning. Just wiping the sheets with a tissue showed that the foulingwas very easily removed from the coated sheets, contrary to the uncoatedsheets. By using water jet all fouling except for one or two smallpatches could be removed from the Coat 1 coated surface. On the Coat 2coated surface some more fouling was present after water jet cleaning.This fouling had the appearance of slightly burnt oil.

Some loss of coating was observed in the contact points but overall thecoated surface that had been in contact with the crude oil was in a goodcondition.

On the sea water facing side both coatings had deteriorated and could bepeeled of quite easily.

TABLE 4 Coat 1 Coat 2 Non-coated View very little fouling reducedfouling fouling significant compared and widespread Wipe very easy tovery easy to fouling was not with remove fouling remove fouling removedtissue High the sheets most of the fouling even after attempts pressureappeared as new was removed of manual removal water of fouling, still awashing considerable layer remains

1. A spiral heat exchanger comprising a spiral body formed by at leastone spiral sheet wound to form the spiral body forming at least a firstspiral-shaped flow channel for a first medium and a second spiral-shapedflow channel for a second medium, wherein the spiral body is enclosed bya substantially cylindrical shell being provided with connectingelements communicating with the first flow channel and the second flowchannel wherein the spiral heat exchanger is provided with a coatingcomprising silicon oxide, SiO_(x), having an atomic ratio of O/Si>1, acontent of carbon ≧10 atomic % and a coating layer thickness of about5-60 μm, which coating was prepared by sol-gel processing and applied toat least a part of the sheets.
 2. A spiral heat exchanger according toclaim 1, wherein the sheets have a thickness of 2-6 mm.
 3. A spiral heatexchanger according to claim 1, wherein the layer thickness of saidcoating on the sheets is 5-30 μm.
 4. A spiral heat exchanger accordingto claim 1, wherein the coating comprising silicon oxide, SiO_(x), hasan atomic ratio of O/Si≧1.5.3.
 5. A spiral heat exchanger according toclaim 1, wherein the composition has a content of carbon ≧20-60 atomic%.
 6. A spiral heat exchanger according to claim 1, wherein the layerthickness of said coating on the sheets is 2-20 μm.
 7. A spiral heatexchanger according claim 1, wherein the coating comprising siliconoxide, SiO_(x), has an atomic ratio of O/Si≧2-2.5.
 8. A spiral heatexchanger according to claim 1, wherein the composition has a content ofcarbon ≧30-40 atomic %.